The present invention is directed to refractory plates of the type having extending therethrough at least one throughflow opening. More particularly, the present invention is directed to such refractory plates for use in slide closures for metallurgical vessels. As employed herein, the term "slide plate" refers not only to plates which move rectilinearly, but also to plates which are used in rotary closures and which undergo a rotary movement.
Such slide plates are employed in various metallurgical industries as a valve-type arrangement for allowing a liquid metal melt to be discharged from a metallurgical vessel, or alternatively to close off such discharge. Such slide plates are generally manufactured at the present time of a mullite-corundum material, generally including more than 80% by weight of alumina and ceramically bound at high temperatures. Additionally, it is known to manufacture such slide plates by the use of ceramically bound magnesite. Even further, it is known to form slide plates of a refractory concrete material which is hydraulically bound by the use of a cement having a high alumina content.
Additionally, slide plates formed of combinations of different materials are known. For example, in German DT-OS No. 1,935,424, there is disclosed a slide closure having a slide plate which is embedded in a ceramic support element which has thermal insulation properties. Also, German Pat. No. 1,937,742 discloses a slide plate formed of two materials, specifically highly heat resistant, hard metallic materials or composite metallic materials of molybdenum and zirconium oxide. In this German patent there is provided an insert of hard material which contacts the liquid metal melt. Such insert should be of a hard material which has a high thermal conductivity which results in warming of the sealing surface, to thereby prevent freezing of the melt when the slide is closed. In such prior art arrangement, a slide plate is formed of a high alumina-containing ceramically bound material. The slide plate has a recess into which is adhered by means of an elastic cement the insert of hard material.
According to the results of recently conducted tests and measurements, refractory plates made of a single material are subject to a destruction mechanism as a result of shock-type increases in temperature in the area of the throughflow opening during the liquid metal melt pouring operation. This results in considerable tangential tensile stresses occurring in the material of the plate at positions a few centimeters away from the throughflow opening. Thus, the plate cracks in such areas in directions radially of the throughflow opening, and such cracking can be more or less visually observed. Thereafter, when the slide closure, including the refractory plate, is moved to a closed position, the closing surface portion of the slide plate, which cuts off the jet of liquid metal melt, is suddenly heated from a previous relatively lower temperature (i.e., on the order of approximately 500.degree. to 800.degree. C.) to a much higher temperature, i.e., on the order of 1500.degree. C. This rapid temperature change leads to relatively high tensile stresses occurring in the material of the plate in an area up to a few millimeters below the surface of the plate which is contacted by the melt. This stressing of such surface will eventually result in elliptical or cup-shaped chipping or peeling of the surface of the plate which is contacted by the melt during the closed position of the slide closure.
Additionally, when the closure is repeatedly opened and closed, or placed in a jet throttling position, an erosion-type washing out occurs along the edge of the throughflow opening. Additionally, particles of steel and slag will gain access to the area between the plates of the slide closure. Such particles will thereafter solidify and will result in erosion of the plate surfaces that slide against each other. Even further, the plates will be subjected to the corrosion action of FeO, acid and slag originating within the melt.
From the above discussion of the recently recognized stressing phenomenon to which the refractory slide plate is subjected, it will be apparent that it would be extremely desirable if the refractory slide plate has the following properties:
(1) A high resistance or durability to cracking;
(2) a high resistance or durability to peeling or chipping;
(3) a high resistance to erosion; and
(4) a high resistance or stability against chemical corrosion.
At the present time, no single material is capable of satisfactorily meeting all of the above four requirements in an economically satisfactory manner. Standard refractory slide plate materials, such as alumina and magnesite, are in some respects satisfactory. However, such customary materials are only moderate to poor with regard to other of the above requirements. For example, magnesite plates have a poor resistance to cracking, while mullite-corundum plates have only a moderate crack resistance and a moderate resistance to chemical corrosion.
With regard to the above noted German Pat. No. 1,937,742, which discloses a plate formed of two materials, the insert discloses in such German patent is a powder metallurgical insert which does not meet all of the above four requirements. Particularly, the powder metallurgical insert of such German patent has only a moderate density and thus offers a poor resistance to erosion. Therefore, and in view of the fact that powder metallurgical inserts are expensive to manufacture, such known slide plate is relatively costly when compared with its expected service life. This is particularly true when considering the fact that the high alumina-containing material, within which the insert is embedded, is likewise a costly material to manufacture. Further, the purpose of using the powder metallurgical insert of such German patent is to produce a controlled transfer of heat from the throughflow opening to that portion of the slide which is subjected to freezing of the melt when the slide is in the closed position.